1. Field of the Invention
The present invention relates to a digital image reading apparatus for reading an original image using a solid-state image pickup element such as a charge-coupled device (to be abbreviated to as a CCD hereinafter) and, more particularly, to adjustment of, e.g., optical characteristics in a digital image apparatus.
2. Related Background Art
In a conventional slit exposure type image reading apparatus using a line sensor such as a CCD, most adjustment methods are performed by using jigs in the production line.
A conventional adjustment method will be described below with reference to FIGS. 1 and 2. In FIG. 1, optical elements for guiding an original image to a CCD mainly include a full-speed mirror unit 93 consisting of a slit 91, a first mirror 92, and an illumination device (not shown), a half-speed mirror unit 96 consisting of second and third mirrors 94 and 95, and an image forming unit 100 consisting of a focusing lens 97 for focusing an original image on the CCD at a reduced scale, a CCD 98, and a CCD driver 99.
In adjustment, points to be noticed are not only fundamental points regarding the optical element such as the magnification, resolving power of a lens, and the like, but also adjustment items inherent to slit exposure. The inherent items include right-angle precision of a central line Q of the slit 91 with respect to the scanning direction (a direction P in FIG. 1) of the full-speed mirror unit 93, parallelism between the central line Q of the slit and a longitudinal axis R of the CCD 98, and coincidence (to be referred to as synchronism adjustment hereinafter) between the central line Q of the slit and the longitudinal axis R of the CCD in the short-side direction of the slit through the optical elements.
In the adjustment method in the production line, a jig chart 101 shown in FIG. 2 is held at the position of a contact glass (not shown), and is exposed from the back surface side to form an image of the jig chart 101 on the CCD 98.
In this case, the following method is normally employed. CCD output signals are processed by a personal computer 102, so that the resolving power is adjusted using a portion A on the chart 101, the magnification is adjusted based on the interval of a portion B, the synchronism adjustment is performed based on a tapered chart C, and the right-angle adjustment is performed based on a portion D, while monitoring a monitor 103.
However, in the prior art, when one of optical elements constituting an optical path for focusing original reflection light onto a CCD by a focusing lens through a plurality of mirrors must be replaced due to some trouble on the market, since there is no jig for the production line, it is difficult to perform adjustment unless the jig chart 101 (including back surface exposure means), the processing personal computer 102, the monitor 103, and the like can be used. For this reason, a portion to be adjusted must be searched little by little while observing images. Thus, maintenance on the market is not considered upon design.